What is the density of the gold
brick?
ȡ = m/V
ȡ = 19.3 kg/0.00100 m^3
ȡ = 19,300 kg/m^3
13.3 - Pressure
Pressure: Final exams,
SATs, free throws in the last
30 seconds of a tight game,
and driver’s license tests.
Pressure: Force divided by
the surface area over which
the force acts.
The first definition of pressure above speaks for itself,
but the second deserves further explanation.
You experience pressure when you swim. If you dive
deep under the water, you can feel the water pushing
against you with more pressure, more force per
square meter of your body.
For a surface immersed in a fluid, the amount of pressure at a given location in the fluid
is the same for any orientation of the surface. If you place your hand thirty centimeters
underwater, the pressure on your palm is the same no matter how you rotate it. In the
aquarium illustrated on the right, the water exerts a force on the bottom of the tank, but
it exerts force í and pressure í on the sides as well.
Pressure equals the amount of force divided by the surface area to which it is applied.
As the photograph above shows, some animals, such as the lynx, are able to travel
easily across the surface of snow because their large paws spread the force of their
weight over a large area. This reduces the pressure they exert on the snow, enabling
them to walk on its surface. Animals of similar weight, but with smaller paws, sink into
the snow.
People who need to travel across the snow may likewise use snowshoes or skis to
increase surface area and reduce the pressure they exert. In contrast, spike-heeled
shoes concentrate almost all the weight of their wearers over a very small surface area,
and can exert a pressure large enough to damage wood or vinyl floors.
The water in the aquarium exerts force on the walls of the tank as well as its bottom. This is shown in Concept 1. Why does the water exert a
force on the sides of the aquarium? Consider squishing down on a water balloon: The balloon bulges out on its sides. The additional force you
exert on the top is translated into a force on the sides. To return to the aquarium, the downward weight of the water results in a force on the
walls as well as the bottom.
The formula in Equation 1 shows how to calculate pressure. It equals the magnitude of the force divided by the area and is a scalar quantity.
The SI unit for pressure is the newton per square meter, called a pascal (Pa). One pascal is a very small amount of pressure. The Earth’s
atmosphere exerts about 100,000 pascals of air pressure at the planet’s surface. A bar of pressure equals 100,000 (10^5 ) pascals and is
another commonly used unit. Bars are informally called “atmospheres” (atm) of pressure. You may have heard references to “millibars” in
weather reports.
In the British system, force is measured in pounds. Pounds per square inch, psi, is a common measure of force. At the Earth’s surface, typical
Big feet distribute this lynx's weight over a larger surface area,
lowering the pressure they exert on the snow.Pressure
Force divided by surface area